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Old 31st Mar 2019, 2:27 pm   #1
G8HQP Dave
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Default "Smoothing" capacitors.

Split from this thread:-

https://www.vintage-radio.net/forum/...d.php?t=155310



'Main filter caps' , which are often electrolytics and mostly reservoir are caps not filters at all.

Personally I never call the big capacitors in a PSU 'filter caps'. The first one is a reservoir cap. Subsquent ones are either smoothers or decouplers; unlike the first one they do have a filtering action. 'Main filter caps' is something you might see more often on an American website.

Last edited by Station X; 1st Apr 2019 at 9:54 am. Reason: Thread split.
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Old 31st Mar 2019, 2:42 pm   #2
ms660
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Default Re: 'X' rated capacitors

I mainly use the words reservoir and filter for the HT capacitors.

Lawrence.
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Old 1st Apr 2019, 9:45 am   #3
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Default Re: 'X' rated capacitors

The reservoir capacitor definitely does act as a filter- there's a lot less ac on the dc line with it present than without it. That makes it a low pass filter even if not in the classic sense. In any case, there's always the rectifier/transformer/source resistance feeding it, so it's still part of an RC filter circuit of sorts.

Reservoir and smoothing are useful terms to specify which of the main filter caps is meant in an equipment, ones "further down the line" generally being called HT decoupling caps.
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Old 1st Apr 2019, 10:51 am   #4
G8HQP Dave
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Default Re: "Smoothing" capacitors.

I disagree. The reservoir cap is a reservoir of charge, and is best understood and calculated as such. The action of the rectifier+cap sub-circuit is too nonlinear to be called a filter. For most of the AC cycle the cap is not connected to the transformer so it is only an RC circuit for part of the time. Newbies trying to treat it as a filter will come badly unstuck.
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Old 1st Apr 2019, 11:14 am   #5
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Default Re: "Smoothing" capacitors.

Hi

My two-penn'worth.

A capacitor is just that, any pre-fixes are merely a foible. If analysed correctly they will reveal what action they are performing. So analysis as a filter in the frequency domain should reveal the same result as an analysis in the time domain. If not - your analysis is wrong! The capacitor doesn't know what it should be doing in circuit, you can call it Daisy and it will still just do exactly what physics tells it to do.

Another example of misleading but purportedly 'helpful' titles is the so-called 'current limiting resistor' which is in fact a 'resistor' it doesn't limit current (just see what happens when you increase the voltage across it) - what it does is simply get hot and dissipate power, analysis will reveal that it follows ohms law.

Cheers
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Old 1st Apr 2019, 11:20 am   #6
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Default Re: "Smoothing" capacitors.

Quote:
Originally Posted by jamesinnewcastl View Post
Another example of misleading but purportedly 'helpful' titles is the so-called 'current limiting resistor' which is in fact a 'resistor' it doesn't limit current (just see what happens when you increase the voltage across it) - what it does is simply get hot and dissipate power, analysis will reveal that it follows ohms law.
Can we stick to "smoothing" capacitors please and avoid another thread split.
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Old 1st Apr 2019, 11:23 am   #7
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Default Re: "Smoothing" capacitors.

Rather analogous to the pressure reservoir in a pneumatic system, you can hear the pump topping up the pressure in short bursts.

Adrian
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Old 1st Apr 2019, 11:50 am   #8
Craig Sawyers
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Default Re: "Smoothing" capacitors.

It is alas wrong that the smoothing capacitor's charging currents are in short bursts. The effective transformer secondary resistance and capacitor value act as a low pass filter with a frequency of a few Hz to low tens of Hz. That is independent of whether it is a valved supply or a transistor supply. That means the peaks in the 100Hz charging current are typically only 3 to 4 times the load current.

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Old 1st Apr 2019, 11:52 am   #9
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Default Re: "Smoothing" capacitors.

The current bursts or pulses depend on transformer type, capacitance and load current. A low voltage toroid maybe be much worse than an HT transformer with E I core as the E I core may have inherent airgaps, the toroid has no airgaps.
Too large a value increases RFI* and can even exceed pulse current ratings or power ratings on semiconductors as the peak power and voltage drop is higher. Valve data sheets often indicated the maximum capacitance for the one on the cathode. However it's a guideline as it also depends on supplied AC voltage, transformer effective series resistance and HT load current.

* Which is why some bridges have a 1nF or similar capacitor on each diode. The 1N4007 is slower than a 1N4001 etc so usually doesn't need the "snubber" capacitor.
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Old 1st Apr 2019, 12:07 pm   #10
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Default Re: "Smoothing" capacitors.

Diode snubber capacitors are the wrong solution. What causes RFI is the reverse recovery transient of the diodes interacting with the transformer interwinding capacitance and leakage inductance. That forms a resonant circuit with a frequency of mid hundreds of kHz (typically) and a high Q - so it rings every time a bridge diode turns off. Interferes with medium wave radio a treat. In fact a way to demonstrate this ringing effect is to poke a wire up the external aerial socket of a medium wave radio and waft it around an operating power supply.

The solution is to forget the capacitors across the bridge, and series RC snub the transformer secondary. That is a 100% solution.

I'm not sure what EI mains transformers have inherent airgaps that you refer to. Most have interleaved laminations to prevent just that happening. (Smoothing chokes *do* have airgaps by design, because they have to cope with a DC current without the core saturating. But that is not the subject of this discussion)

There is a good article that covers the transformer snubber, and explains the problem rather well here http://www.hagtech.com/pdf/snubber.pdf

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Old 1st Apr 2019, 9:39 pm   #11
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Default Re: "Smoothing" capacitors.

....................................



Quote:
Originally Posted by G8HQP Dave View Post
.......... The reservoir cap is a reservoir of charge, and is best understood and calculated as such.
I don't disagree with that, but it's still a filter removing ac signal from the dc line.


The action of the rectifier+cap sub-circuit is too nonlinear to be called a filter. For most of the AC cycle the cap is not connected to the transformer so it is only an RC circuit for part of the time. Newbies trying to treat it as a filter will come badly unstuck.
Just because the filter treatment maths is non trivial doesn't stop it being a filter. But yes, for a simple estimate of p-p ripple voltage, treating the capacitor as a charge store with a constant current load is quick and easy.

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Old 2nd Apr 2019, 6:31 am   #12
Craig Sawyers
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Default Re: "Smoothing" capacitors.

I went back to the source - O. Schade's seminal paper ("Analysis of Rectifier Operation", Proc IRE, July 1943, pp341-361) on power supply design. He drew the curves that most paper and pen power supply designers use today.

He does not use either "filter", "reservoir" or "smoothing".

The terms he uses are "Choke input", "Condenser input" and "Choke input with capacitance-shunted resistive load" to describe the three basic topologies. He provides theory, "oscillograms" to prove the theory works, and provides graphs to allow calculation for half-wave, full wave and voltage doubler rectifiers for each topology.

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Old 2nd Apr 2019, 8:04 am   #13
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Default Re: "Smoothing" capacitors.

But here is another paper, by General Radio in 1964. They use the term "filter" in relation to what follows the rectifier in a power supply:

http://www.ietlabs.com/pdf/GR_Appnot...r%20Design.pdf

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